docs: def next_update(self):
          """Compute the next expected update date,
  
          given the frequency and last_update.
          Return None if the frequency is not handled.
          """
          delta = None
          if self.frequency == 'daily':
              delta = timedelta(days=1)
          elif self.frequency == 'weekly':
              delta = timedelta(weeks=1)
          elif self.frequency == 'fortnighly':
              delta = timedelta(weeks=2)
          elif self.frequency == 'monthly':
              delta = timedelta(weeks=4)
          elif self.frequency == 'bimonthly':
              delta = timedelta(weeks=4 * 2)
          elif self.frequency == 'quarterly':
              delta = timedelta(weeks=52 / 4)
          elif self.frequency == 'biannual':
              delta = timedelta(weeks=52 / 2)
          elif self.frequency == 'annual':
              delta = timedelta(weeks=52)
          elif self.frequency == 'biennial':
              delta = timedelta(weeks=52 * 2)
          elif self.frequency == 'triennial':
              delta = timedelta(weeks=52 * 3)
          elif self.frequency == 'quinquennial':
              delta = timedelta(weeks=52 * 5)
          if delta is None:
              return
          else:
              return self.last_update + delta